Certain large homes require more heating or cooling than can be provided by a single residential HVAC unit. Instead of installing a larger commercial HVAC unit, installers may install two residential furnaces side by side with shared common ducting. This effectively increases the amount of heat that can be distributed into the home. Both furnaces operate from one thermostat, and ideally operate in the same mode at the same time. Such an arrangement is referred to as a twinned system. One aspect of a twinned system is that the circulator blowers should operate at the same speed. If one HVAC system operates its blower at a different speed than the other HVAC system, there would be different pressure drops across the units that would cause air from one unit to be forced into the other unit. This different pressure condition can cause problems if one unit's blower operation depends on a safety switch, or a different time delay period than the other unit.